The Centers for Disease Control have determined just how off this year’s batch of flu vaccines was, with more than three-quarters of Americans who got the shot still at risk of getting the virus anyway, given a mismatch between the flu strains covered by the shot and those actually causing illness in people.
An interim CDC report found the shot was only 23 percent effective overall, about in line with its predictions last year, experts said. At the time, CDC warned that the predominant flu virus, influenza A (H3N2), had mutated since the shot was made.
Effectiveness varies widely by age, working best in young, healthy people and least well in the elderly, a pattern reflected in the report released Thursday. It showed effectiveness against H3N2 viruses was highest (26 percent) among children aged 6 months to 17 years. It was roughly 12 percent effective in people 18-49 and 14 percent effective in those 50 and older, but those estimates were not statistically significant because there was too little data at this point in the flu season.
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"These estimates of vaccine efficacy are doleful, and they are entirely consistent with what the CDC told us they were likely to be based on the studies in the lab," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville.
Since the CDC started doing flu vaccine studies in the 2004-2005 flu season, overall effectiveness has ranged from 10 percent to 60 percent.
The results reflect limitations in the current shots, in which flu strains for the Northern Hemisphere are selected in late February of the prior year, giving manufacturers roughly six months to make the vaccines from scratch before the start of the flu season in August.